Megasonic processing apparatus with frequency sweeping of thickness mode transducers

ABSTRACT

A megasonic processing apparatus and method has one or more piezoelectric transducers operating in thickness mode at fundamental resonant frequencies of at least 300 KHz. A generator powers the transducers with a variable-frequency driving signal that varies or sweeps throughout a predetermined sweep frequency range. The generator repeatedly varies or sweeps the frequency of the driving signal through a sweep frequency range that includes the resonant frequencies of all the transducers.

RELATED APPLICATION

This application is a continuation-in-part application of U.S.application Ser. No. 10/983,183, filed Nov. 5, 2004, now U.S. Pat. No.7,247,977 entitled ULTRASONIC PROCESSING METHOD AND APPARATUS WITHMULTIPLE FREQUENCY TRANSDUCERS, and invented by J. Michael Goodson. Thisapplication also claims priority from co-pending U.S. ProvisionalApplication No. 60/783,213, filed Mar. 17, 2006, entitled MEGASONICPROCESSING APPARATUS WITH FREQUENCY SWEEPING, and invented by J. MichaelGoodson. These two prior applications are expressly incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to megasonic processing apparatus andassociated methods involving one or more piezoelectric transducersoperating in thickness mode at megasonic frequencies of at least 300 KHzor higher, and relates more particularly to improving performance bysweeping the frequency of a driving signal throughout a predetermined orprogrammable frequency range that spans the resonant frequencies of allthe transducers.

2. Description of the Relevant Art

Megasonic processing involves generating and using high frequency energyat frequencies above 300 KHz. Many megasonic systems operate atfrequencies at or near 1,000 KHz, or one megahertz. Although 1 MHz isthe consensus, preferred frequency for many applications, the frequencyrange goes much higher, with frequencies as high as 10 MHz. Typical usesfor megasonic systems include cleaning delicate objects, such assemiconductor wafers and disc drive media. Such a megasonic cleaningprocess involves placing the objects to be cleaned in a fluid-filledtank, and applying vibrational energy at megasonic frequencies to aradiating surface or surfaces of the tank. One or more piezoelectrictransducers are used to generate the vibrational energy. A generatorsupplies an alternating current driving signal at the resonant frequencyof the transducers. Megasonic transducers operate in thickness mode,where a piezoelectric element is excited by an alternating currentdriving signal that causes alternating expansion and contraction of thetransducer, primarily expanding and contracting the thickness of thetransducer. A piezoelectric transducer having a thickness of 0.080inches has a fundamental, thickness mode, resonant frequency of 1,000KHz.

Megasonic processing has some similarities with ultrasonic processing,which involves lower fundamental frequencies, typically from about 25KHz to about 192 KHz. Ultrasonic transducers are typicallymass-balanced, with inert masses on either side of a piezoelectricelement, and have a significant radial component of movement at rightangles to the thickness component. One common construction of anultrasonic transducer is to stack several layers of ring-shapedpiezoelectric elements between two masses, and to hold the assemblytogether with an axial compression bolt. Ultrasonic cleaning is based oncavitation, which is the formation and collapse of bubbles in the fluid.

At the frequencies used for megasonic cleaning, significant cavitationdoes not occur, so the cleaning action is based on another mechanismknown as micro-streaming, which is a general flow of detached particlesflowing away from the megasonic transducers. This flow consists ofplanar waves originating at the surface to which the transducers aremounted. The planar nature of these micro-streams affects thedistribution of megasonic energy throughout the tank. One way to improvethe distribution is to cover a high percentage of the surface area ofthe tank with transducers. Another but less efficient way is tooscillate or move the parts to be processed throughout the tank so thatall surfaces are exposed to sufficiently high megasonic energy.

It is known that radial-mode ultrasonic activity in a cleaning tank maybenefit from a process of sweeping or varying the frequency of thedriving signal. However, there has been an industry-wide belief that youcannot sweep megasonic frequencies because the sound waves are too smalland weak for any benefit from sweeping. In addition, it has been thoughtthat there would be no benefit from sweeping megasonic frequenciesbecause of the thickness mode transducers and resultant planar nature ofmegasonic vibrations and due to the different cleaning mechanisms atwork as compared to ultrasonics.

SUMMARY OF THE INVENTION

The present invention relates to a megasonic processing apparatus andmethod having one or more piezoelectric transducers (PZT) operating inthickness mode at megasonic frequencies in excess of 300 KHz. Amegasonic generator operating at megasonic frequencies drives thetransducers with a variable-frequency driving signal that varies orsweeps throughout a predetermined or programmable sweep frequency range.The megasonic generator generates the driving signal at megasonicfrequencies to energize the megasonic piezoelectric transducers to causethem to vibrate in thickness mode at their megasonic resonantfrequencies. The piezoelectric transducers emit energy at the megasonicfrequencies that can be used for various applications, such as cleaningobjects in a fluid-filled tank. The generator repeatedly varies orsweeps the frequency of the driving signal through a sweep frequencyrange that includes the resonant frequencies of all the megasonicpiezoelectric transducers.

Another aspect of the present invention involves grouping the megasonicpiezoelectric transducers into groups having similar resonantfrequencies, and powering each group with a separate frequency-sweepingdriving signal from a generator operating within a sweep frequency rangethat includes the resonant frequencies of the group of associatedtransducers. This subdivides the overall sweep frequency range intosmaller subranges, which may or may not overlap, and reduces the rangeof each frequency sweep. The effect of grouping transducers is toproportionately increase the amount of time that any particulartransducer is operating at or close to its resonant frequency andthereby improve efficiency.

The present invention encompasses a megasonic system that includes oneor more piezoelectric transducers and one or more megasonic generatorscoupled to the transducers for supplying varying-frequency megasonicdriving signals at selectable or programmable frequency ranges and sweeprates.

When a megasonic process is used, for example, for cleaning siliconwafers or disc drive media, sweeping the driving signal through theresonant frequencies of all the thickness-mode megasonic transducerswill equalize the megasonic energy generated by the transducers and willcause the transducers to perform in unison. This results in a moreuniform distribution of megasonic energy and improved performance. Thesame improved megasonic energy uniformity and functionality can also beachieved in liquid processing, non-destructive testing, medical imaging,and other processes using megasonic thickness-mode transducers bysweeping the range of resonant frequencies of the transducers. Thefrequency sweeping process will also extend the life of the megasonictransducers because it is less stressful to the transducers thanoperating at a single fixed frequency. The frequency sweeping processalso improves the uniformity of megasonic energy throughout the tank orother apparatus because each transducer operates at its resonantfrequency during at least part of each frequency sweeping cycle. It isexpected that any application or process using megasonic frequencieswill benefit from the uniform distribution of power created by sweepingthe driving signal through all the transducers' resonant frequencies.

A key to optimizing efficiency of a megasonic process is to have uniformenergy throughout the radiating surface being excited with megasonics.To do this, preferably 80% or more of the area of the radiating surfaceis covered by thickness-mode megasonic transducers. Furthermore, eachmegasonic transducer produces consistent megasonic energy by sweepingthe frequency of the driving signal through the highest and lowestresonant frequencies of a group of transducers.

For best performance, each megasonic transducer needs to be energizedsubstantially the same as other megasonic transducers bonded to the samesurface. To achieve this, the driving frequency is swept through theresonant frequencies of all the transducers. Sweeping the resonantfrequencies of megasonic transducers drives every transducer at itsresonant frequency at some point in each cycle. This creates uniformityin transducer performance not previously achieved in the industry.

In addition, frequency sweeping of megasonic transducers reduces a“fountain effect” observed with fixed-frequency megasonic transducers.The fountain effect is thought to be caused by a transducer operating atits resonant frequency with a fixed frequency driving signal, whichproduces a significant up-surge of liquid in the tank above thattransducer. Sweeping the megasonic frequency driving signal ensures thatany particular transducer will not be driven continuously at itsresonant frequency, thus eliminating the upsurge associated with thefountain effect. Instead, the megasonic energy is uniformly distributedthroughout the tank because all transducers are operating efficiently attheir resonant frequencies at some point during each sweep cycle.

Frequency sweeping is far more dramatic with megasonic frequencies thanultrasonic frequencies like 40 KHz. Improvements in power distributionof 500 to 700% have been seen with megasonic resonant frequency sweepingand this means substantially better processing.

The features and advantages described in the specification are not allinclusive, and particularly, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification and claims hereof. Moreover, it should be notedthat the language used in the specification has been principallyselected for readability and instructional purposes, and may not havebeen selected to delineate or circumscribe the inventive subject matter,resort to the claims being necessary to determine such inventive subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall perspective view of a megasonic processing systemaccording to the present invention.

FIG. 2 is a top perspective view of a tank used in the megasonicprocessing system of the present invention.

FIG. 3 is a bottom perspective view of the tank.

FIG. 4 is a side elevation view of the tank.

FIG. 5 is a bottom view of the tank.

FIG. 6 is a schematic view of the megasonic processing system and asectional view of the tank and an attached megasonic transducer with agenerator that supplies driving signals to the transducer for creatingmegasonic vibrations in liquid in the tank.

FIG. 7 is a graph of frequency versus time of a driving signal used inone embodiment of the present invention.

FIG. 8 is a graph of frequency versus time of two driving signals usedin another embodiment of the present invention in which the sweep periodis the same as in FIG. 7.

FIG. 9 is a graph of frequency versus time of two driving signals usedin another embodiment of the present invention in which the sweep rateis the same as in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drawings depict various preferred embodiments of the presentinvention for purposes of illustration only. One skilled in the art willreadily recognize from the following discussion that alternativeembodiments of the structures and methods illustrated herein may beemployed without departing from the principles of the inventiondescribed herein.

One aspect of the present invention is a megasonic processing apparatusand method having a megasonic generator with a programmable sweepfrequency range and a programmable sweep rate. The sweep frequency rangeis the range of frequencies or bandwidth within which the megasonicgenerator outputs a driving signal to drive one or more megasonicthickness-mode piezoelectric transducers at their resonant frequencies.The sweep rate is the number of times the resonant frequencies are sweptper second.

The megasonic generator preferably includes a controller or othercontrolling device with means to allow a user to select or program thesweep frequency range or bandwidth and the sweep rate for the drivingsignal. The user inputs one or more combinations of sweep frequencyrange and sweep rate into the memory device of the generator. Thegenerator generates and outputs the driving signal according to thesweep frequency range and sweep rate selected by the user.

When used in a cleaning application, for example, the megasonicpiezoelectric transducer or transducers may be mounted on the bottom orsides of a tank, or enclosed in an immersible container within the tank.The sweeping frequency generator may be used to drive megasonictransducers in applications other than cleaning. Preferably, thetransducers are piezoelectric crystals or piezoelectric ceramic (alsoknown as PZTs), such as barium titanate or lead zirconate titanate,operating in thickness mode. Using different sweep rates or sweepfrequency ranges in the same process may enhance cleaning of some partsbecause certain frequencies may be more effective than others.

A device that sweeps the frequency of the driving signal is incorporatedinto the megasonic generator that generates the driving signal. Thegenerator includes a user interface that includes one or more inputdevices, such as knobs, dials, software, keyboard, graphical userinterface, network connection, or other input devices, that permit auser to set a sweep frequency range or bandwidth over which thegenerator operates and also to set a sweep rate at which the generatorsweeps through the programmed range. The controls for user programmingthe sweep frequency range and sweep rate may be analog or digital.

As shown in FIGS. 1-6, one embodiment of the present invention is acleaning system 10 that includes a quartz cleaning tank 12 containing acleaning liquid or solution 14 and one or more pieces 15 to be cleaned.Megasonic energy is supplied to the cleaning liquid 14 by one or moremegasonic frequency transducers 16 affixed to the bottom of the tank 12.Alternatively, megasonic transducers could be affixed to one or moresides of the tank or immersed in the tank. Preferably, the megasonictransducer 16 has a piezoelectric element (PZT) 18 adhesively bonded orotherwise attached to one side of a silicon carbide plate 20. The otherside of the silicon carbide plate 20 is adhesively bonded or otherwiseattached to the outside bottom surface of the cleaning tank 12.Preferably, bonding layers 22 between the silicon carbide plate 20 andthe tank 12 and between the silicon carbide plate and the piezoelectricelement 18 are composed of perforated copper foil and an impedancematching adhesive. Alternatively, the bonding layers may be composed ofepoxy or other adhesive used for die bonding semiconductor chips topackage substrates.

The piezoelectric element can be square, rectangular, or a circulardisk, or other shape having uniform thickness. For example, foroperation at a nominal frequency of 1,000 KHz, the piezoelectric element18 would have a thickness of about 0.08 inches, the silicon carbideplate 20 would have a thickness of about 0.19 inches, and the bottom ofthe quartz tank 12 would have a thickness of about 0.20 inches.Transducer 16 and cleaning system 10 is just one example of a transducerand apparatus that incorporates the present invention.

As shown in FIGS. 3-6, the transducers 16 are preferably rectangular inshape and are arranged parallel to each other. Preferably, thetransducers 16 cover a substantial portion of the bottom surface of thetank 12, preferably at least 80%. It is desirable to generate megasonicenergy and transfer it to the tank 12 and fluid 14 uniformly throughoutthe entire area of the surface to which the transducers 16 are attached.Covering a high percentage of the surface area of the tank bottom withtransducers ensures that the megasonic energy transferred to the fluid14 is relatively uniform.

As shown in FIG. 6, the transducers 16 are driven by a driving signalsupplied over electrical wires 24 by a programmable generator 26. Thegenerator 26 is programmed by a user through a user input or interface28 to set the sweep frequency range or bandwidth and the sweep rate ofthe driving signal output by the generator.

A megasonic frequency piezoelectric transducer operates in thicknessmode such that applied voltages cause the transducer to expand andcontract in thickness. These expansions and contractions are transmittedthrough the silicon carbide resonator 20 and tank 12 to the fluid 14 andobjects 15 in the tank. As shown in FIG. 6, these megasonic-frequencyvibrations are primarily horizontal waves 17, provided that thetransducers 16 are on the bottom of the tank 12. The waves propagateupwards and convey particles cleaned or separated from the objects 15 inthe tank. This is a processed known as micro-streaming, in which thereis a net movement upward, away from the source of megasonic energy. Asshown in FIGS. 1 and 2, the tank has a weir 21 over which excess fluidand particles flow, and a pump 23 and filter 25 to recirculate and cleanthe fluid.

Resonant frequency is generally the frequency where the mechanical andelectrical properties of a transducer can most efficiently transmitsound waves. In megasonic transducers operating in thickness mode, thethickness of the transducer determines the resonant frequency. Forexample, a transducer that is 0.08 inches thick will have a resonantfrequency of about 1,000 KHz. A transducer that is 0.065 inches thickwill have a resonant frequency of about 1230 KHz. A transducer that is0.050 inches thick will have a resonant frequency of about 1600 KHz. Theterm “resonant frequency” is used herein to mean the lowest, fundamentalfrequency where the transducer as installed has a natural resonance.

As stated above, a piezoelectric transducer having a thickness of 0.080inches has a fundamental resonant frequency of 1,000 KHz. A tolerance onthe thickness of such a transducer has a significant effect on theresonant frequency. A thickness variation of 0.001 inch would cause aresonant frequency variance of 12.5 KHz. Also, the two major surfaces ofthe transducer should be flat and co-planar, but any variances can alsoaffect the resonant frequency. Even though it is desirable from aperformance standpoint for all transducers to have exactly the sameresonant frequency, from a manufacturing tolerance standpoint, it isimpractical. However, the frequency sweeping of the present inventionovercomes this obstacle.

One advantage of the present invention is that sweeping the frequency ofthe driving signal through the resonant frequencies of all thetransducers distributes the sound waves equally among the transducers.This makes it possible to have substantially equal megasonic energythroughout the tank. This is important because the thickness-modetransducers produce sound waves that travel vertically from the bottomto the top of the tank with little spreading in lateral directions. Theeven distribution of megasonic energy can best be achieved by sweepingjust outside the highest and lowest resonant frequencies of thetransducers.

Another advantage of the present invention is that it accommodatestolerances in the resonant frequencies of the transducers. Performanceis best if variances of the resonant frequency are minimized. Choosingtransducers with exactly the same resonant frequency will help minimizevariances (although at increased cost), but even then there will be somevariances from the adhesives or other binder materials used to mount thetransducers because any variation in thickness creates a variation infrequency with thickness mode applications. Sweeping the frequency ofthe driving signal according to the present invention accommodates suchinevitable variations.

Still another advantage of the present invention is that it reducessurges of fluid in the tank. Without sweeping the driving signal, thetransducers at or closest to the frequency of the driving signal tend tocreate a powerful upward force that pushes the fluid upward, sometimesas much as two inches above the surface level. Such surface surges are aproblem because they cause air to be incorporated into the fluid as itis recirculated, which can interfere with the megasonic process. Surgesare also a problem because if the liquid is solvent it will evaporate inthe air and can be harmful to the operator and or the people in thearea, especially if the fluid is an acid or other hazardous material.Sweeping the driving signal with the present invention reduces theseproblems.

As shown in FIG. 7, the generator 26 varies the frequency of the drivingsignal as a function of time. For example, the frequency of the drivingsignal may vary linearly in a saw-tooth pattern over a programmed sweepfrequency range 30 that includes the resonant frequencies 31 of all themegasonic transducers 16. The sweep frequency range or bandwidth of thegenerator is programmed by a user and stored in a memory deviceassociated with the generator 26. The rate at which the frequency variesis determined by the sweep rate programmed by the user and stored in thememory device of the generator. The generator can be programmed to varythe frequencies of the driving signal according to other functions orprograms and need not be limited to linear functions that form atriangular wave or saw tooth pattern as shown in FIG. 7. The variationin frequency can be, for example, sinusoidal, exponential, and otherfunctions. The driving signal itself may be sinusoidal, square,triangular, or other wave shape. The sweep rates need not be the samefor sweeping upwards (increasing frequency) and downwards (decreasingfrequency). Preferably, the user can also set the number of periods andcan establish rest times when the generator shuts off the drivingsignal.

In a cleaning application, some parts may be best cleaned by a singletransducer instead of multiple transducers. In such a configuration, theperformance of the transducer can be enhanced by using a programmedsoftware program that identifies the optimum resonant frequency andsweeps through this frequency within a defined range. For best results,the driving frequency can be swept through a sweep range of 1% or lessto ensure that the resonant frequency of the transducer is being excitedrepeatedly. A benefit of the present invention is that it reduces theadverse effects of resonant frequency drifting because the resonantfrequency of each transducer is being excited each cycle even if itchanges with time, provided that the sweep range or bandwidth is wideenough.

Commonly, multiple megasonic transducers 16 are used for a given task orprocess, in which case it is common to drive all transducers with thesame generator and driving signal. Where multiple transducers are used,however, there may not be a single optimum frequency due to performancevariations and manufacturing tolerances among the transducers.Production tolerances result in megasonic transducers having resonantfrequencies within a 3% to 4% range. For example, at 1000 KHz, a 4%range would be plus or minus 20 KHz from the nominal 1000 KHz, or arange of 980 to 1020 KHz.

In such applications, according to the present invention, it isappropriate to repeatedly sweep the frequency of the driving signal toensure that at least some of the time the transducer 16 is operating ator near its resonant frequency. In order to have each transducer 16operate at or near its resonant frequency, the generator sweeps througha predetermined sweep frequency range that is designed to reach thelowest and highest resonant frequencies 31 of the group of transducers.The sweeping frequency function of the generator 26 covers that range ofvariance. The frequency sweeping function can be fixed or it can beprogrammed to be variable as to speed (sweeps per second) or range(minimum and maximum frequencies).

Another aspect of the present invention relates to grouping themegasonic piezoelectric transducers into multiple groups according totheir resonant frequencies, and driving each group with a separatevariable-frequency driving signal. Transducers with similar resonantfrequencies are grouped together to reduce the range of frequenciesthrough which the generator must sweep in order to operate the group oftransducers at or near their resonant frequencies. Reducing thefrequency range of the sweep increases the time that each transduceroperates at or near its resonant frequency.

As the range of sweep frequency coverage is reduced, the rate of sweepcan be increased to create more activity if required by a particularapplication, or if the sweep rate remains the same, then the repetitionrate is increased. The result is that the megasonic transmission at eachtransducer's resonant frequency will be greater since the sweep covers ashorter span and the transducer operates for a greater percentage oftime at or near its resonant frequency, which increases the efficiencyof the megasonic process.

This point is illustrated in FIGS. 7, 8, and 9. In FIG. 7, a singlegenerator sweeps the driving signal between minimum and maximumfrequencies over a range 30. In FIG. 8, two generators are used to coverthe same overall range, but each generator covers a subrange 32 that isone-half of the full range 30. Half of the transducers have resonantfrequencies 31′ in the upper subrange 32′, and the other half of thetransducers have resonant frequencies 31″ in the lower subrange 32″. Thenumber of sweeps per unit time is the same in FIGS. 7 and 8. In FIG. 9,the rate of change of the sweeping frequency is the same as in FIG. 7,but the range is cut in half so that twice as many sweeps occur in thesame period of time.

As an example of grouping, assume that twelve megasonic transducers areused in a process having the following nominal resonant frequencies (inKHz):

1010 1030 1015 1007 1019 1004 1027 1038 1022 1014 1031 1040These frequencies range from a minimum of 1004 KHz to a maximum of 1040KHz, for a total range of 36 KHz (±18 KHz) centered at 1022 KHz.Sweeping the frequency of the driving signal to include the resonantfrequencies of all twelve transducers would require a total sweep of 36KHz.

These twelve transducers could be divided into two groups, A and B, toreduce the sweep range:

Generator A Generator B 1004 1014 1022 1031 1007 1015 1027 1038 10101019 1030 1040The transducers driven by Generator A range from 1004 KHz to 1019 KHz,for a total range of 15 KHz (±7.5 KHz) centered at 1011.5 KHz. Thetransducers driven by Generator B range from 1022 KHz to 1040, for atotal range of 18 KHz (±9 KHz) centered at 1031 KHz. By grouping thetransducers according to their resonant frequencies and reducing thesweep range for each sweeping generator, the number of sweeps per unittime can be increased or the sweep rate can be decreased, either ofwhich allows the transducers to be driven at or near their resonantfrequencies more often, which enhances the megasonic process.

In actual practice, the sweep frequency ranges are set slightly outsidethe maximum and minimum resonant frequencies for the associatedtransducers. So, in the example above, the sweep frequency range ofGenerator A might be set to 1003 to 1020 KHz or 1002 to 1021 KHz and thesweep frequency range of Generator B might be set to 1021 to 1041 KHz or1020 to 1042 KHz. This ensures that each transducer operates both belowand above its resonant frequency in each frequency sweep cycle and alsoallows for shifts of the resonant frequencies that may occur due toheating or other variables.

Transducers can be grouped within an individual system or process oramong multiple systems or processes operating simultaneously. Forexample if there are two tanks with multiple transducers each and bothtanks will be used simultaneously, one can group transducers from thelarger universe of all transducers on the two tanks. Groupings may befurther selected to produce a more uniform result as the transducerspowered by a single generator do not have to be next to each other orused with the same tank to be in the group. Because all transducers worksimultaneously, the designer of the transducer layout can focus onmaximizing the efficiency of the grouping without regard to where themembers of the groups are located.

As an example of grouping among multiple, simultaneous processes, assumethat the same twelve megasonic transducers set forth in the previousexample are located on two different tanks:

Tank 1 Tank 2 1010 1030 1015 1007 1019 1004 1027 1038 1022 1014 10311040

The twelve transducers of Tanks 1 and 2 are divided into two groupsaccording to resonant frequencies and are driven by Generators A and Bas follows (with the tank number shown in parentheses):

Generator A Generator B 1004 (1) 1014 (1) 1022 (1) 1031 (2) 1007 (2)1015 (2) 1027 (2) 1038 (2) 1010 (1) 1019 (1) 1030 (1) 1040 (2)Generator A drives four transducers of Tank 1 and two transducers ofTank 2. Generator B drives two transducers of Tank 1 and fourtransducers of Tank 2. Since all transducers are operating at the sametime, this grouping allows the two generators to sweep across smallerranges.

Thus, in cleaning and other processes where multiple tanks or systemsare used, the entire population of transducers in multiple tanks orsystems can be combined to create an optimum assortment of frequenciesto be grouped together, with each group powered by a different sweepinggenerator. For example in four processes using four tanks, transducersfrom any or all of the four tanks may be networked together to achievethe optimum range of frequencies for sweeping. Of course, all processesmust be active at the same time for such grouping.

Another aspect of the present invention is the construction of themegasonic transducer 16 and its attachment to another structure, such asthe bottom of tank 12, using a perforated metal layer and impedancematching adhesive. As shown in FIGS. 4 and 6, the megasonic transducer16 preferably has a silicon carbide plate 20 between the piezoelectricelement 18 and the surface of the cleaning tank 12 or other structure towhich the transducer is attached. The piezoelectric element 18 is bondedto the silicon carbide plate 20, and the assembly is bonded to the tank12 with bonding layers composed of a perforated metal foil, preferablycopper, and an adhesive.

The perforated copper (or other metal) foil improves flatness anduniformity of thickness of the bonding layer 22. The perforated copperhas a predetermined thickness that allows the adhesive to be evenlydistributed, thus avoiding irregularities or non-uniformity of adhesivethickness without using a jig or other stabilizing device. Theperforated metal provides a controllable flat structure to maintainuniformity in thickness of the adhesive. The perforated metal alsoserves as an electrode between the piezoelectric element and the siliconcarbide plate.

The application of the present invention is not limited to cleaningoperations. The same principle of sweeping the acoustical energy formegasonic transducers can be applied to other uses of micro-streaming ofmegasonic energy, such as non destructive testing, or any otherapplications using thickness mode transducers having fundamentalresonant frequencies of at least 300 KHz. Sweeping megasonic transducerscreates greater energy bursts, which create improved and strongermicro-streaming activity which improves the efficiency ofmicro-streaming cleaning and other uses of micro-streaming.Micro-streaming is a flow of energized liquid created by the release ofultrasonic energy that is too weak to cause cavitation. At frequenciesin excess of 300 KHz, cavitations cease to exist but the megasonicfrequency energy creates a flow of the liquid.

From the above description, it will be apparent that the inventiondisclosed herein provides a novel and advantageous megasonic processingapparatus and method utilizing a variable frequency driving signal. Theforegoing discussion discloses and describes merely exemplary methodsand embodiments of the present invention. As will be understood by thosefamiliar with the art, the invention may be embodied in various otherforms without departing from the spirit or essential characteristicsthereof. Accordingly, the disclosure of the present invention isintended to be illustrative, but not limiting, of the scope of theinvention, which is set forth in the following claims.

1. A megasonic processing apparatus comprising: one or more unclampedpiezoelectric transducers, each having a fundamental resonant frequencyin thickness mode vibration of at least 300 KHz; an unpressurized tankadapted to contain liquid and one or more parts to be processed, whereinsaid one or more transducers provide megasonic vibrations to thecontents of the tank; and a generator coupled to the transducers forsupplying a driving signal at a variable frequency throughout afrequency range that includes the fundamental resonant frequencies ofall the transducers.
 2. An apparatus as recited in claim 1 wherein thegenerator has an adjustable sweep rate and an adjustable frequencyrange.
 3. An apparatus as recited in claim 2 wherein the sweep rate isin the range of 50 to 1200 sweeps per second.
 4. An apparatus as recitedin claim 1 wherein the apparatus has at least four transducers and twogenerators, wherein the transducers are grouped by similar resonantfrequencies, and wherein each group of transducers is powered by aseparate generator that generates a driving signal having a variablefrequency that varies within a frequency range that includes theresonant frequencies of all the transducers of its associated group. 5.An apparatus as recited in claim 1, wherein the one or more transducersare affixed to a surface of the tank.
 6. A megasonic processing systemcomprising: two or more unpressurized tanks, each tank adapted tocontain liquid and one or more parts to be processed; one or moreunclamped piezoelectric transducers associated with each tank, eachtransducer having a fundamental resonant frequency in thickness mode ofat least 300 KHz; wherein the transducers provide megasonic vibrationsto the fluid and contents of the tank; and two or more generatorscoupled to the transducers for supplying driving signals to thetransducers, wherein the transducers are grouped by similar fundamentalresonant frequencies, and wherein each group of transducers is poweredby a separate generator that generates a driving signal having avariable frequency tat varies within a frequency range that includes thefundamental resonant frequencies of all the transducers of itsassociated group.
 7. A megasonic cleaning apparatus comprising: one ormore piezoelectric transducers, each having a fundamental resonantfrequency in thickness mode vibration at a frequency of at least 300KHz; an unpressurized tank adapted to contain liquid cleaning fluid andone or more parts to be cleaned, said one or more transducers providemegasonic vibrations to the cleaning fluid and parts in the tank; and agenerator coupled to said one or more transducers for supplying adriving signal at a predetermined frequency range and sweep rate,wherein the frequency range includes the fundamental resonant frequencyof all said one or more transducers, and wherein the generator includesprogrammable means for defining a sweep frequency range and a sweep ratefor the driving signal.
 8. A megasonic processing method comprising:providing one or more unclamped piezoelectric transducers, each having afundamental resonant frequency in thickness mode vibration of at least300 KHz; providing an unpressurized tank adapted to contain liquid andone or more parts to be processed, wherein said one or more transducersprovide megasonic vibrations to the contents of the tank; and generatingand supplying a driving signal to the transducers, wherein the drivingsignal has a variable frequency throughout a frequency range thatincludes the fundamental resonant frequencies of all the transducers. 9.A megasonic processing apparatus comprising: one or more unclampedpiezoelectric transducers, each having a fundamental resonant frequencyin thickness mode vibration of at least 300 KHz; and a generator coupledto the transducers for supplying a driving signal at a variablefrequency throughout a frequency range that includes the fundamentalresonant frequencies of the transducers.